The invention relates to a deflection unit for a color television display tube, which deflection unit has a field deflection coil, a line deflection coil, and an annular member of soft-magnetic material surrounding at least the line deflection coil. A line deflection coil is to be understood to mean, in this context a combination consisting of two diametrically oppositely arranged coil portions for deflection an electron beam in a first (horizontal) direction, and a field deflection coil is to be understood to mean, in this context a combination consisting of two diametrically oppositely arranged field coil portions for deflecting an electron beam in a (vertical) direction, transverse to the first direction. Each deflection coil portion may be of the saddle type and may consist of electrical conductors which are wound so as to form a first and a second side strip, a front and a rear end which together define a window, at least the front end being constructed as an upright edge (flange), the line and field deflection coils being surrounded by the annular member of soft-magnetic material (the core); or the line deflection coil portions may be of the saddle type and the line deflection coil may be surrounded by the core, while the field coil portions are wound toroidally on the core, this latter case being a hybrid system.
For displaying (color) television pictures, certain electro-optical requirements are imposed upon the combination of the display tube and the electron beam deflection device.
It holds, for example, that the raster reproduced on the display screen must be rectangular and undistorted within certain narrow limits. Furthermore the definition of the picture from the center towards the edge of the screen may decrease only to a restricted non-disturbing extent.
For the color display tube having a shadow mask there are two additional requirements.
The color selection in a shadow mask tube is obtained by an eccentric arrangement of the three electron guns in such manner that the phosphor dots of a given color are hit only by the electrons of the corresponding beam through the holes in the mask. In order to obtain a color-pure image it is required that the relative color selection angles of the three beams should remain unvaried upon deflection. This is the landing requirement. When this condition is not satisfied, it is possible that color spots will occur.
A second equally important requirement is that the targets of the three electron beams should coincide with each other throughout the screen so that the pictures in the three primary colors fully converge. This is the convergence requirement. When this condition is not satisfied, disturbing color edges at brightness and color transitions occur.
Of great importance in the further development of color television display systems was the introduction of the "in-line gun" display tube in which the electron guns are arranged in one plane. The basic idea of this design is that it must be possible with this arrangement to obtain automatic convergence (self-convergence) throughout the display screen while using astigmatic deflection fields. A correct astigmatism level for the field deflection coil will be described hereinafter.
Fora good astigmatism level for the field deflection coil, its magnetic field should show a barrel-shaped variation in the middle and on the screen side of the deflection unit. If this variation is realised with a set of conventional (straight-wound) toroidal field deflection coil portions or with a set of conventional saddle-shaped field deflection coil portions (having a constant average window opening), then this means necessarily that the produced magnetic field has a barrel-shaped variation everywhere, so also on the gun side. "Straight wound" is understood to mean herein that the turns constituting the coil portions are located in planes passing through the longitudinal axis of the core. since it is usual to position the three electron guns in the sequence red, greed, blue, this has for its result that, during the deflection, the green beam lags with respect to the average of the red beam and the blue beam. This deflection error is termed coma.
In itself it is possible to mitigate coma by winding the field deflection coil portions in a special manner: for this purpose, a tororidal field deflection coil portion should be wound "obliquely", and a saddle-shaped field deflection coil portion should be wound so that the average window opening varies in the axial direction.
However, the disadvantage of this solution is that, apart from the more complicated winding process, it introduces substantial East-West raster distortion.